A new process for preparing sodium carboxy-methyl cellulose.
The present invention relates to a process and an equipment for preparing sodium carboxy-methyl cellulose.
When preparing NaCMC and CMC, the cellulose is pretreated mechanically and chemically before the etherification itself. The purpose of the mechanical treatment is to achieve accessibility and homogeneity, and by means of the chemical treatment, the cellulose is activated with NaOH. As a result of the pretreatment, i.e. the mercerization, alkali cellulose is produced, which forms NaCMC together with the etherification reagent.
In sheet mercerization, the pretreatment is based on submerging whole cellulose sheets in an 18% NaOH water solution. The amount of NaOH remaining in the sheets is regulated by compression of the sheets, the problem then being that the NaOH content will be too large with regard to an economical accomplishment of the reaction. Only long-fibred cellulose qualities can be successfully used in the process because of the disintegration of the sheets. The excess NaOH removed by compression is difficult to neutralize because of the impurities suspended and dissolved from the pulp.
In slurry methods, cellulose is finely ground and treated with a dissolvent and NaOH. As reaction medium, a large number of various dissolvents are used, as for instance isopropyl alcohol or acetone. In the slurry method 10-25 1 of solvents is typically used per kg of cellulose, often requiring the use of solid NaOH for the mercerization of the cellulose. Besides being expensive, the treatment of solid NaOH on an industrial scale is extremely difficult because of its great hygroscopicity. The reaction being accomplished at a thin consistency, large reactors are additionally needed in order to achieve a certain capacity. Moreover, a great distilling capacity is required for regenerating the dissolvent.
In certain slurry methods, the cellulose has to be ground to as low a particle size as 200 .mu.m, this being slow and difficult for maintenance. The slurry method has been used in the U.S. Pat. No. 3,284,441 and the FI patent specification 71750, among others.
Sheeted cellulose can also be dispersed in a shredder and pneumatically transferred into the reactors, to which the reaction medium and an aqueous, about 50% NaOH solution is subsequently added, by means of which the cellulose is transformed into reactive alkali cellulose (Finnish Chemical Industry, Chemical Industry Federation of Finland, p. 149).
In the kneader, i.e. breaking methods, the mercerization takes place at a consistency corresponding to 1-6 1/kg of the dissolvent amount. In methods of the kneader type, the cellulose is not ground as finely as in slurry methods. Thus, the cellulose is unhomogeneous, and difficult to mix, so that the method poses extremely high requirements as to the reactor technique to be used. In less expensive reactor types, large amounts of chemicals have to be used in order to produce end products of good quality.